backgrounds and sensitivity expectations for...

Backgrounds and Sensitivity Expectations for XENON100

IDM08, Stockholm, August 19, 2008

Laura BaudisUniversity of ZurichFor the XENON100 Collaboration

LNGS collaboration meeting, Oct. 20071

Laura Baudis, University of Zurich, IDM08, Stockholm, August 19, 2008

The XENON Program

XENON R&D

XENON10

XENON100

XENON1t

???

ongoing

2006-2007

in progress

2009-2013

2


Reminder: Backgrounds in XENON10

• Dominated by contribution from detector materials:• steel (~ 180 kg, 60Co), PMTs (89 R8520, U/Th/K/Co) and ceramic HV feed-throughs (U/Th/K)

2 cm radial cut (+ z-cut) → 5.4 kg LXe mass=> background rate of 0.6 events/(kg d keV)

Data

Monte Carlo simulation

Fiducial mass here: 8.9 kg

dru = events/(kg day keV)

3


The XENON100 Detector and Background Goals

• Factor ~100 reduction compared to XENON100:• 1 dru -> 10 mdru raw gamma BG

How?• selection of ultra-low background

materials for detector components and shields

• active LXe veto shield (100 kg)surrounding target on all sides

• reduce intrinsic 85Kr contaminationto the required level (50 ppt)

• detector design: place cryogenics and feed-throughs outside the Pb/Poly shield

XENON10 XENON100

4


• Factor ~100 reduction compared to XENON100:• 1 dru -> 10 mdru raw gamma BG

How?• selection of ultra-low background

materials for detector components and shields

• active LXe veto shield (100 kg)surrounding target on all sides

• reduce intrinsic 85Kr contaminationto the required level (50 ppt)

• detector design: place cryogenics and feed-throughs outside the Pb/Poly shield

• improve shield by reducing poly activity with 5 cm of OFRP Cu

The XENON100 Detector and Background Goals

5 cm Cu on door 5 cm Cu top/sides

5


Xenon100 Materials and ShieldsPolish Lead: 15 cm, 27 t

French Lead: 5 cm, 6 t

Polyethylene: 20 cm, 2 t

Copper: 5 cm, 2 t

Columbia student

UZH postdoc

UZH postdoc

LNGS student

Veto PMTs and holders (Cu)TPC structure (PTFE)(and PMTs inside)

Bell (stainless steel)

Cryostat (stainless steel)

Support bars:stainless steel

6


Xenon100 Materials and Shields

• The shield is continuously flushed with N2 to avoid events related to radon decays inside it• The radon levels are continuously monitored (all written out to slow control)

Xenon100 BG rate, June 6-9, 2008radon detector (RAD7)

inlet

Rn concentration

Trigger rate

Rn c

once

ntra

tion

[Bq/

m3 ]

Correlation between Rn values and DAQ rate

7


XENON100 Material Screening

• Ultra-low background, 100 % efficient (2 kg) HPGe-spectrometer, operated at LNGS (plus detectors from LNGS screening facility)

• Shield: 5 cm of OFRP Cu (Norddeutsche Affinerie); 20 cm Pb (Plombum, inner 5 cm: 3 Bq/kg 210Pb), air-lock system and nitrogen purge against Rn, slow control for online monitoring of HV, N2 flow rate, leakage current and LN level

• Background spectrum: < 1 event/(kg d keV) above 40 keV

• Screened all XENON100 detector/shield components for a complete BG model

Data: stainless steel sample

Background30 kg days (October 2007)

coun

ts/k

g/d

/keV

60Co lines

Energy [keV]

8


XENON100 Material Screening

Material* 238U 232Th 40K 60Co

Stainless Steel 1.5 mm (316Ti, Nironit; cryostat)

Stainless Steel 25 mm (316Ti, Nironit, cryostat)

PMTs (R8520-AL)

PMT Bases

Teflon (TPC)

Poly I (shield)

Poly II (shield)

Polish Pb (outer shield)

French Pb (inner shield)

<2 mBq/kg <2 mBq/kg 10.5 mBq/kg 8.5 mBq/kg

<1.3 mBq/kg <0.9 mBq/kg <7.1 mBq/kg 1.4 mBq/kg

< 0.24 mBq/PMT 0.18 mBq/PMT 7.0 mBq/PMT 0.67 mBq/PMT

0.16 mBq/pc 0.10 mBq/pc <0.16 mBq/pc <0.01 mBq/pc

< 0.3 mBq/kg <0.6 mBq/kg < 2.3 mBq/kg --

< 3.8 mBq/kg < 2.9 mBq/kg < 5.88 mBq/kg --

2.43 mBq/kg < 0.67 mBq/kg <4.66 mBq/kg --

< 5.7 mBq/kg < 1.6 mBq/kg 14 mBq/kg < 1.1 mBq/kg

< 6.8 mBq/kg < 3.9 mBq/kg < 28 mBq/kg < 0.9 mBq/kg

* only a selection is shown here, all PMTs are screened and show consistent values; also screened: copper, cables, screws, ...** thanks also to Matthias Laubenstein (LNGS screening facility)

9


Gamma Background Predictions from MC Simulations

Material Rate [mdru]

Stainless steel (cryostat, 65 kg) 2.01 ± 0.22

Teflon (TPC, 10.7 kg) 0.18 ± 0.02

PMTs (including bases, 242) 4.91 ± 0.60

Polyethylene (shield, 2t) 2.50 ± 0.29

Copper (shield, 2t) 0.026 ± 0.002

Total* 9.63 ± 0.70

* dominant background rate before S2/S1 discrimination in fiducial mass dru = events/(kg day keV)

10


Gamma Background Predictions from MC Simulations

Total single scatters in fiducial volume

11


Neutron Backgrounds: MC Simulations

• Internal neutron BG from detector materials + shield, from (α,n) and fission reactions

• Numbers based on detailed MC, with measured U/Th activities of all materials

MaterialTotal

n-rate [yr-1]Single NR rate [μdru]

Stainless steel 17.7 0.29

Teflon 28.0 0.58

PMTs 7.0 0.32

LXe 0.81 0.007

Copper 1.6 0.01

Poly shield 416.3 0.49

Polish Pb 5805.8

French Pb 416.3 0.38

Total ~ 1.74

=> ~ 0.6 single NRs/year in FV(~ 44% of events are singles)

Steel: for 10 ppb U

Poly: for 10 ppb U

12


WIMP rate versus neutron background

• assumptions:➡ spin-independent WIMP-nucleon cross section: 2x10-45 cm2

➡ local WIMP density: 0.3 GeV/cm3

100 GeV WIMP

neutron background

13


Preliminary Background from XENON100 Data

data (S1 only) Monte Carlo simulations

Data and Monte Carlo predictions are in good agreement for overall rate

light yield: 2.6 pe/keV

14


Expected Sensitivity for WIMPs and SUSY Predictions

WIMP Mass [GeV/c2]

Cro

ss-s

ecti

on [

cm

2]

(norm

ali

sed t

o n

ucle

on)

101

102

103

104

10-46

10-45

10-44

10-43

10-42

10-41

Spin-independent Spin-dependent (pure n-couplings)

Many SUSY models CMSSM

XENON10

XENON100

XENON100

XENON10

XENON10 limit: arXiv: 0805.2939accepted in PRL08

XENON10 limit: PRL100, 021303 (2008)

15


Expected sensitivity for heavy Majorana Neutrinos

XENON100

XENON10

XENON100:expected number of events in 50 kg x 300 days

XENON10:expected number of events in 5.4 kg x 58.6 days⇒MνM < 9.4 GeV and > 2.2 TeV

arXiv: 0805.2939accepted in PRL08

16


Expected Sensitivity for WIMPs and UED Predictions

LHC reach in 4l+ET channel

WMAP5 region(WIMPs make 100% of the dark matter)

Spin-independent Spin-dependent

S. Arrenberg, L. Baudis, K.C. Kong, K. Matchev, J. Yoo, arXiv:0805.4210accepted in PRD08see talk by S. Arreberg

XENON100

XENON100

17


Conclusions and Outlook

• XENON100 is in commissioning phase at LNGS➡ strong effort to reduce backgrounds by factor 100 compared to XENON10

• electromagnetic background: dominated by PMTs, stainless steel cryostat and poly shield➡ room for improvement (lower-activity poly, Cu cryostat, lower BG PMTs)

• nuclear recoil background dominated by internal neutrons from material➡ less than 1 single NR/year predicted for ~ 46 kg fiducial mass➡ muon induced NR background under study (expected to be negligible)

• preliminary data from XENON100 => overall background as expected based on MCs• extrapolated sensitivity to WIMPs is ~ 2x10-45 cm2 for SI couplings for ~7 months exposure

• XENON100 has a fantastic discovery potential for dark matter particles of galactic origin and can test many theoretical models of particle physics beyond the Standard Model

18


End

19


Neutron background from the rock and concrete

⇒ (5.0 ± 1.0) single nuclear recoils/year in 63 kg LXe⇒ (2.5 ± 0.9) single nuclear recoils/year in 46 kg LXeadditional water/PE shield outside the Pb is being added to reduce this background component to negligible levels

U/Th activities taken from measurements of LNGS Hall A (similar to XENON10 location)

20


Neutrons from Materials

• single nuclear recoils from neutrons generated by (alpha,n) and fission reactions in detector and shield materials

21


Next Step: XENON1t

• Studies in progress for 3 ton (1 ton fiducial) LXe detector• Possible location: inside LVD SN neutrino detector at LNGS -> active veto for µ-induced neutrons• Gamma flux inside LVD structure: 10-20 times lower than in main halls (detailed mapping of gamma

and neutron background in progress)

1m

1m 22

backgrounds and sensitivity expectations for...

Documents